Formation damage in openhole sand control completions comes from a variety of sources.The drill in fluid filtrate can cause near wellbore permeability alteration and in the case of a gravel packed completion, leak off from the gravel pack slurry can increase the damage. Once the gravel is in place, poor filter cake clean up and pack damage can further increase the skin.Careful design and execution of the gravel pack can result in a low skin.
However there is one additional damage mechanism, which is impossible to avoid, that of permeability alteration by rock failure.There is a high probability that rock failure takes place at some stage in the life of an openhole sand control completion and the effect of rock failure on the skin depends on the change in permeability produced by failure and on the completion type.
An objective of this work was to measure the effect of rock failure on permeability for a weak sandstone.
A yield zone model was used to determine the depth of rock failure away from the wellbore and the effective stresses in the yield (failed) zone. The permeability changes were coupled with the yield zone model to predict the evolution of skin during depletion for a variety of completion options. Using the properties for the rocks tested, the model predicts for a compliantly expanded expandable sand screen completion an initially zero or slightly negative skin that increases slightly as depletion proceeds.For a non-compliant screen the model predicts a slightly higher initial skin, which again increases slightly with depletion.The predicted values are commensurate with observed skin values for openhole expandable sand screen completions.
There are many types of openhole sand control but they can be divided into two types: those that support the original wellbore, and those that don't. The type that support the wellbore are referred to as compliant; these include openhole gravel packs and compliant expandable sand screens.The non-compliant type includes the various designs of stand-alone screen and non-compliant expandable screens.
Compliant and non-compliant sand control systems each have their advantages and disadvantages.The compliant systems support the wellbore and remove the annulus.Under certain conditions this improves long-term productivity and reliability. Non-compliant sand control, in the form of stand-alone screens used in the proper application, also provide excellent sand control, productivity and reliability.Compliant sand control in the form of openhole gravel packs tends to be operationally more complex but is suited to the most difficult applications.
The influence of compliance on near wellbore permeability has the potential to influence skin and productivity.This is shown in this paper by using a theoretical derivation of the depth of failure around a wellbore with the inclusion of measured permeability changes during deformation.
This investigation was prompted by differences in the damage skin observed in fully compliant and fixed size expandable sand screen(ESS®.) installations.